The atomic structure and electronic and vibrational properties of glassy Ga 11 Ge 11 Te 78 have been studied by combining density functional (DF) simulations with x-ray (XRD) and neutron diffraction (ND), extended x-ray absorption fine structure (EXAFS), and Raman spectroscopies. The final DF structure (540 atoms) was refined using reverse Monte Carlo methods to reproduce the XRD and ND data as well as Ge and Ga K-edge EXAFS spectra, while maintaining a semiconducting band gap and a total energy close to the DF minimum. The local coordination of Ga is tetrahedral, while Ge has twice as many tetrahedral as defective octahedral configurations. The average coordination numbers are Ga, 4.1, Ge, 3.8, and Te, 2.6. The chemical bonding around Ga involves Ga 4s, Ga 4p, Te 5s, and Te 5p orbitals, and the bond strengths show bonding close to covalent, as in Ge. There are fewer Te chains and cavities than in amorphous Te, and a prepeak in the structure factor at 1.0Å −1 indicates medium-range order of the Ga/Ge network. Density functional calculations show that contributions of Te-Te, Ga-Te, and Ge-Te bonds dominate the experimental Raman spectra in the 110-150 cm −1 range.

Density functional/molecular dynamics simulations have been performed to determine structural and other properties of amorphous Ag/Ge/S and Ge/S alloys. In the former, the calculations have been combined with experimental data (x-ray and neutron diffraction, extended x-ray absorption fine structure). Ag/Ge/As alloys have high ionic conductivity and are among the most promising candidates for future memristor technology. We find excellent agreement between the experimental results and large-scale (500 atoms) simulations in Ag/Ge/S, and we compare and contrast the structures of Ge/S and Ag/Ge/S. The calculated electronic structures, vibrational densities of states, ionic mobilities, and cavity distributions of the amorphous materials are discussed and compared with data on crystalline phases where available. The high mobility of Ag in solid state electrolyte applications is related to the presence of cavities and can occur via jumps to a neighbouring vacant site.

Density functional/molecular dynamics simulations and experimental data (x-ray and neutron diffraction, extended x-ray absorption fine structure) have been combined to determine structural and other properties of amorphous AsS 2 and AgAsS 2 . These semiconductors represent the two small regions of the Ag-As-S ternary diagram where homogeneous glasses form, and they have quite different properties, including ionic conductivities. We find excellent agreement between the experimental results and large-scale (over 500 atoms) simulations, and we compare and contrast the structures of AsS 2 and AgAsS 2 . The calculated electronic structures, vibrational densities of states, ionic mobilities, and cavity distributions of the amorphous materials are discussed and compared with data on crystalline phases where available. The high mobility of Ag in solid state electrolyte applications is coupled to the large cavity volume in AsS 2 and local modifications of the covalent As-S network in the presence of Ag.

Localization and coordination of Mg2+ cations in Mg-FER were investigated by a combination of IR spectroscopy of CO probe molecule and periodic DFT calculations employing the ω/r correlation for calculations of CO stretching frequencies. Experimental data were interpreted on the basis of a very good agreement between theoretical and experimental results. The most intense band in the spectra centered at 2205 cm−1 was assigned to monocarbonyl complexes formed on Mg2+ cations located in the six-member ring separating two adjacent FER cages (P6 site). Dicarbonyl complexes can be formed on these Mg2+ sites (characterized by a band at 2194 cm−1) where each of the CO molecules approaches Mg2+ from a different FER cage. The band at 2211 cm−1 observed for the intermediate CO coverage was assigned also to the monocarbonyl complexes on Mg2+ sites inside the FER cage (P6 and P8 sites); the difference between 2205 and 2211 cm−1 bands is due to the different geometry of Al pair within the ring. At very low CO coverages, IR spectra show a band at 2222 cm−1 assigned to carbonyl complexes on Mg2+ cations at the channel intersection and in the main channel. Because of a significant difference in the intensity of the bands at 2222 and 2211 as compared to that of the band at 2205 cm−1, it is concluded that Mg2+ cations exchange preferably into the six-member ring between two FER cages. An unusually high site-specificity of νCO observed for CO complexes formed in Mg-FER can be exploited in the investigation of preferable exchange sites of Mg2+ in zeolites and in the investigation of the localization of Al pairs in zeolites.